1. Field of the Invention
The present invention relates to a method for compensating x-ray imaging systems for radiation scatter.
2. Description of the Prior Art
In x-ray imaging systems, the total flux of detected radiation consists not only of photons that did not interact with the elements of the attenuating object being imaged but also of radiation scatter. Specifically for systems using area detectors the amount of radiation scatter can be very large. For a large class of applications, such as energy-selective imaging, this radiation can be an important source of error and must be compensated in order to have satisfying results.
A lot of work has been done already in searching good methods for compensating x-ray imaging systems for radiation scatter.
The use of grids or air gaps reduces but doesn't eliminate the radiation scatter. For many applications, as for dual x-ray absorptiometry (DXA), it is not sufficient.
Several analytical models, representing the scatter--some of them use point spread functions--are proposed in the prior state of the art. They require parameters of which the values are difficult to obtain and mostly to be found experimentally. These models do not give satisfying accuracy in some applications.
In the prior state of the art, some investigators (Molloi SY, Mistretta CA. Scatter-glare correction in quantitative dual-energy fluoroscopy. Med. Phys. 1988; 15:289-297) use correction tables for a specific application that give a hypothesised relationship between the detected grey level in a certain pixel and the scatter fraction. They are for most applications rough estimations for which the accuracy is insufficient.
In the prior state of the art, (Wagner F C, Macovski A, Nishimura D. Dual-energy x-ray projection imaging: Two sampling schemes for the correction of scattered radiation, Med. Phys. 1988;15:732-748), a method is sometimes used in which two x-ray irradiations of an object are made; one with a disk sampler, consisting of an array of small lead disks above the object, and one without the sampler. In the shadow of each disk only scattered radiation is detected and the average of the pixel values in this shadow gives the value of the radiation scatter. By a fit of a low-frequency surface through the sample values of the radiation scatter, an estimation of the radiation scatter in the whole image is generated. The scatter corrected image is obtained by subtracting the scatter-surface from the second image. A disadvantage of this method is that one needs two shots of the object. In medical applications, it means that the patient (=object) receives a larger x-ray dose and that he may have moved between two shots. Switching quickly disk sampler and detectors asks for a mechanically complicated system. Another implementation of the method with the lead beam stops in which only the x-ray irradiation with the disk sampler is made has the disadvantage that all the information about the object is lost under the beam stops. This can be an important disadvantage.
In the prior state of the art, another method for scatter radiation compensation, recently proposed in the literature (Shaw C. A novel technique for simultaneous acquisition of primary and scatter image signals. SPIE Vol. 1651 Medical Imaging VI: Instrumentation (1992), p. 222-233), is the Primary-Modulation-Demodulation (PMD) technique. The primary x-ray distribution is modulated and demodulated with two filters of equal material and thickness placed on the tube and detector sides of the objects. The modulation-demodulation process results in a reduction of scatter signals in selected regions of the image. It leaves the overall primary signal distribution unchanged. The signal drop of the scatter radiation can be measured and used to estimate the scatter radiation signal in the selected regions. Although the PMD method allows both primary and scatter signals to be acquired simultaneously, it has two main disadvantages; it is unknown how the drop in scatter radiation relates to location, scattering geometry, patient, etc.. Another disadvantage is that it is practically impossible to match the modulator and the demodulators. Therefore the results are based on rough estimates and the accuracy is reduced.